Conventionally, a cruising lane recognition device is known that captures an image of an area including the road ahead of a vehicle, and recognizes a cruising lane marker (such as a white lane marker) on the road based on the changes in luminances within the image (see Patent Document 1). The cruising lane recognition device can, for example, be used in an on-board system that issues a warning that the vehicle will deviate from the cruising lane. In other words, the cruising lane recognized by the cruising lane recognition device and a cruising trajectory of the vehicle predicted based on a yaw sensor or a speed sensor can be analyzed, and the risk of cruising lane deviation can be judged.